The present embodiments relate to generating a signal for a transmission antenna in a magnetic resonance imaging system.
Using magnetic resonance imaging (MRI), slice images of the human or animal body may be generated. The slice images permit an assessment of the organs and many pathological organ changes. MRI is based upon very strong magnetic fields generated in a magnetic resonance imaging (MRI) system and alternating magnetic fields in the radiofrequency range, by which specific atomic nuclei (e.g., the hydrogen nuclei/protons) are resonantly excited in the body. As a result of this, an electric signal is induced in a receiver circuit.
MRI systems may have a transmitter that is provided to generate a substantially homogeneous radiofrequency field for exciting the nuclear spins. The associated transmitter antenna, which is also referred to as “body coil”, may be fixedly installed in the magnet and the gradient coils. As an example, a “birdcage” antenna includes a cylindrical form and substantially consists of two rings that are connected to one another using a number of uniformly spaced apart antenna rods arranged in parallel. Connection points of the antenna rods on the rings are connected to one another via a capacitor. The capacities of the capacitors are selected such that the antenna arrangement is resonant at the examination frequency (e.g., between 60 and 125 MHz).
For the spatial resolution of the signals, the frequency and phase encoding is imaged in the pulse sequences transmitted by the transmission antenna. Therefore, a corresponding module for generating variations in frequency and in phase is provided in a corresponding signal generation module arranged upstream of the transmission antenna. The module for generating variations in frequency and in phase actuates a digitally controlled oscillator and generates the corresponding vibrations. The baseband data generated in a baseband module are modulated by the variations in frequency and in phase and the radiofrequency carrier signal. This may take place in complex number space (e.g., the individual signals are produced as real and imaginary parts and modulated by multiplication).
The generated modulated signal, which may be a single-sideband modulation (SSB) signal, is transmitted to an amplifier (e.g., radiofrequency power amplifier, RFPA). The RFPA amplifies the signal and transfers the signal to the transmission antenna. For linearization purposes, a control loop may be embodied with the aid of a directional coupler (DICO) and a coherent demodulation. This control loop acts on the generated baseband data and may correct not only the amplitude, but also the phase of the SSB signal.
Depending on the adaptation of the transmission antenna, the power is reflected back into the RFPA. The RFPA is to be configured for this reflected power since there is a superposition of the forward-directed wave and the reflected wave. Alternatively or additionally, with the aid of a circulator, the reflected power may be directed to a load instead of into the RFPA. A circulator is a component that routes the power in a circular manner from one port to the next. As a result, the reflected power is no longer fed back to the RFPA, but rather into a load.
The nonlinearities created by the circulator may likewise be corrected in this case (e.g., by the temporal properties thereof or the properties that change with the temperature). This is because a circulator changes properties during the operation in a manner characterized by the scattering parameters. It is therefore desirable for the control loop to also register the nonlinear and time-changing properties (e.g., depending on other variables such as temperature) of the circulator (e.g., for the circulator to be part of the control loop).